This invention relates generally to wireless communications systems. More particularly, it relates to a novel method and system for distributing multiband wireless communication signals.
As wireless communications become a way of life in society, a challenge to wireless communications network operators is to transport and distribute multiband wireless communications signals in an efficient, flexible, and economical manner. And the challenge is particularly acute in areas that are not traditionally covered by macro-networks. Such areas reside mostly in indoor environments, including airports, malls, office buildings, tunnels, hotels, convention centers, and sports arenas.
Distributed radio systems are conventionally used in the art to provide the radio coverage to the indoor environments, employing an architecture of one distributed antenna system supporting one wireless radio frequency (RF) band. Such an architecture entails that in order to support multiple RF bands, separate distributed antenna systems must be installed in parallel, each accommodating a specific RF band. This is a rather inefficient, and at times, cumbersome undertaking.
The past few years have seen a few other approaches in the art, attempting to distribute multiband wireless communication signals in a more efficient manner. For example, U.S. Pat. No. 5,969,837 by Farber et al. describes a communications system in which multiple RF signals from multiple wireless communications networks are first combined at a base unit. The combined RF signal is then split into multiple outputs, which are subsequently converted to optical signals and transmitted to remote units by optical fibers. At each remote unit, the received optical signal is converted back to an RF signal. The RF signal is then split and routed to separate antennas, wherein each antenna is designated to a specific frequency band (e.g., PCS, GSM, or paging).
A notable disadvantage of the above prior art system is that each frequency band requires a dedicated antenna, which handles both downlink and uplink RF signals by way of a duplexer. (And it should be noted that the duplexer (84) as disclosed by Farber et al. cannot feasibly separate more than one frequency band, particularly intertwined bands such as cellular and iDEN bands.) Such a configuration can become considerably bulky and inefficient, especially when dealing with multiple (e.g., more than two) frequency bands. There are additional shortcomings common to the above and other prior art multiband distributed systems, summarized as follows:
1. The prior art systems typically employ a star architecture, in which each remote unit is connected to a main (or base) unit by a dedicated fiber-optic cable. Such an architecture is inflexible and inefficient for many applications.
2. Strong downlink RF signals transmitted by the main unit tend to interfere with the reception of weak uplink RF signals in a remote unit by saturating the front-end radio receivers.
3. Intermodulation products produced by the nonlinearities in the downlink amplifiers tend to fall into the uplink frequency bands, thereby desensitizing the uplink receivers.
4. Intermodulation products produced in one downlink frequency band often fall into other downlink frequency bands, thereby causing regulatory violations.
5. Adjacent and/or intertwined frequency bands (e.g., iDEN and cellular bands) cannot be feasibly separated and therefore effectively filtered and amplified.
6. The prior art systems cannot support Time Division Duplex (TDD) protocols, in which the downlink and uplink RF signals share the same frequency band but are interleaved in time.
7. The prior art systems are devoid of carrying out an end-to-end gain calibration, such that a prescribed gain for each of the frequency bands is established in each of the remote units.
In view of the forgoing, there is a need in the art for a multiband distributed wireless communications system that overcomes the prior art problems.
The aforementioned need in the art is provided by a novel method and system for distributing multiband wireless communication signals according to the present invention. In a multiband distributed wireless communications system of the present invention, a main unit is linked to multiple remote units by optical fibers in a hybrid star/cascaded architecture. As a way of example, the main unit can be directly connected to individual remote units, and/or connected to one or more cascaded chains of remote units. The main unit can also be linked to some of the remote units via one or more expansion units in an hierarchical (or tree-like) structure. Such a hybrid star/cascaded architecture of the present invention provides a modular and flexible way of distributing multiband wireless communications signals, particularly in an indoor environment.
In the present invention, multiband wireless communications signals are transported and distributed as follows. On the downlink, a plurality of downlink RF-sets in a plurality of downlink frequency bands transmitted from a plurality of wireless communication networks are received at the main unit. The downlink RF-sets each contain downlink RF signals in one of the downlink frequency bands. Some of these downlink RF signals are frequency-division-duplexed (FDD), such that downlink and uplink RF signals are separate in frequency; while others are time-division-duplexed (TDD), such that downlink and uplink signals share the same frequency band but are separated in time.
The received downlink RF-sets are then combined into a combined downlink RF signal, which is subsequently split into multiple downlink RF-parts. Each downlink RF-part is essentially a xe2x80x9ccopyxe2x80x9d of the combined downlink RF signal in that it contains the downlink RF signals from all of the downlink RF-sets. The downlink RF-parts are then converted to downlink optical signals in a one-to-one correspondence, which are subsequently transmitted to the remote units by way of optical fibers.
At each of the remote units, a delivered downlink optical signal is converted to a delivered downlink RF-part. The delivered downlink RF-part is then separated into a plurality of downlink RF-groups by frequency band. Individual downlink-signal-conditioning is subsequently performed on each of the downlink RF-groups, wherein the downlink-signal-conditioning includes one or more steps of RF-amplifying, gain-adjusting, and RF-filtering. By performing amplification on the downlink RF-groups separately, nonlinear intermodulation products amongst the downlink RF-groups can be effectively avoided. The individual-conditioned downlink RF-groups are then combined and transmitted to a downlink antenna. Note that in the present invention, each remote unit is in RF-communication with at least one downlink antenna dedicated to handle the downlink RF signals transmitted from the remote unit.
Likewise, each of the remote units is also in RF-communication with at least one uplink antenna dedicated to handle the uplink RF signals to be received by the remote unit. Having separate uplink and downlink antennae enables the reception of uplink RF signals and the transmission of downlink RF signals to occur with spatial separation in the present invention. Such spatial separation creates propagation loss between the transmit (uplink) and receive (downlink) antennae, which helps protect the sensitive uplink receiver from being desensitized by strong downlink RF signals and/or by downlink intermodulation products that fall into one or more uplink frequency bands.
On the uplink, multiple uplink RF signals in a plurality of uplink frequency bands are first received by the uplink antenna connected to the remote unit. The received uplink RF signals are then separated into a plurality of uplink RF-groups by frequency band. Individual uplink-signal-conditioning is subsequently performed on each of the uplink RF-groups, which includes one or more steps of RF-amplifying, gain-adjusting, and RF-filtering. The individual-conditioned uplink RF-groups are then combined into an uplink RF-part, which is further converted to an uplink optical signal. As such, multiple uplink optical signals corresponding to multiple uplink RF-parts are optically transmitted from the remote units to the main unit.
At the main unit, the received uplink optical signals are first converted back to the uplink RF-parts. The uplink RF-parts are then combined into a combined uplink RF signal, which is subsequently transmitted to the wireless communications networks.
The present invention advantageously utilizes various frequency translations to allow for separation of downlink RF signals into downlink RF-groups by frequency band using feasible means (such as RF-filtering), whereby these downlink RF-groups can be individually conditioned (e.g., filtered and amplified) at a remote unit before being transmitted to a downlink antenna. The frequency translations can also be effectively used to prevent the interference effects and intermodulation products amongst different (downlink and uplink) frequency bands. For instance, a first frequency-translation may be performed on one or more downlink RF-sets, so as to place the downlink RF-sets in disjoint frequency bands that are sufficiently far apart to allow for economical separation of downlink RF signals in different frequency bands by RF-filtering. Such a task would otherwise be very difficultxe2x80x94if not entirely impossiblexe2x80x94to accomplish, particularly when dealing with adjacent (and/or intertwined) frequency bands. At each of the remote units, a second frequency-translation may be subsequently performed on one or more downlink RF-groups, which substantially undoes the effect of the first frequency translation and thereby places the downlink RF signals back to their original downlink frequency bands respectively. There can also be first and second frequency-translations performed on one or more downlink RF-groups at a remote unit, whereby the downlink-signal-conditioning (e.g., RF-filtering, and RF-amplifying) on these downlink RF-groups can be performed more effectively in one or more intermediate frequency bands. Similarly, there can be first and second frequency-translations performed on one or more uplink RF-groups at a remote unit, so as to perform the uplink-signal-conditioning on these uplink RF-groups more effectively in one or more intermediate frequency bands. As such, these frequency translations effectively facilitate the transportation and distribution of multiband RF signals, and are particularly desirable when dealing with RF signals in, adjacent (and/or intertwined) frequency bands.
The present invention further entails carrying out an end-to-end gain calibration, thereby setting a prescribed gain for each of the downlink RF-groups. To maintain the prescribed gain over temperature changes and other effects, a downlink-gain-control signal (e.g., a pilot or an Frequency-Shift-Key signal) that is set to a frequency outside of any of the frequency bands used by the wireless communications networks (and frequency-translated bands) can be injected to and transmitted along with each of the downlink RF-parts to the remote units. At each of the remote units, the downlink-gain-control signal is detected and thereby used to maintain the gain for each of the downlink RF-groups at the prescribed level.
In the present invention, the downlink and uplink optical signals between the main unit and remote units can be further transmitted via one or more expansion units. For example, a downlink optical signal can be first transmitted from the main unit to an expansion unit, where it is amplified and further split into multiple secondary-optical-signals. The secondary-optical-signals are then transmitted to additional remote units (and/or one or more lower-level expansion units). On the uplink, a plurality of uplink optical signals from a number of the remote units can be first transmitted to an expansion unit, where they are amplified and further combined to a combined optical signal. The combined optical signal is then transmitted to the main unit (or to a higher-level expansion unit). The deployment of the expansion units enhances the flexibility and efficiency of the present invention in transporting and distributing multiband wireless communication signals.
In an exemplary embodiment of a multiband distributed wireless communications system according to the present invention, the main unit comprises an RF-downlink-interface for receiving a plurality of downlink RF-sets in a plurality of downlink frequency bands from a plurality of wireless communications networks; a downlink RF-combining means for combining the downlink RF-sets into a combined downlink RF signal; a downlink RF-splitting means for splitting the combined downlink RF signal into multiple downlink RF-parts; and multiple RF-to-optical converters for converting the downlink RF-parts to downlink optical signals. The main unit further comprises multiple optical-to-RF converters for converting the received uplink optical signals to uplink RF-parts; an uplink RF-combining means for combining the uplink RF-parts into a combined uplink RF signal; and an RF-uplink-interface for transmitting the combined uplink RF signal to the wireless communications networks.
Each of the remote units comprises a downlink optical-to-RF converter for converting a delivered downlink optical signal to a delivered downlink RF-part; a downlink splitting-filtering means for separating the downlink RF-part into a plurality of downlink RF-groups by frequency band; a plurality of downlink-signal-conditioning assemblies for performing individual downlink-signal-conditioning on each of the downlink RF-groups, and a downlink filtering-combining means for combining the individual-conditioned downlink RF-groups into a downlink RF-transmit signal, which is to be transmitted to a dedicated downlink antenna. The downlink splitting-filtering means can be provided by a series of RF-filters configured in parallel, each characterized by a distinct frequency passband. Each of the downlink-signal-conditioning assemblies can be in the form of one or more RF-amplifiers, gain-adjusting elements, and RF-filters. Note that in the present invention, each of the remote units is in RF-communication with at least one downlink antenna, dedicated to handle downlink RF signals in a plurality of frequency bands.
Moreover, each of the remote units is in RF-communication with at least one dedicated uplink antenna, from which multiple uplink RF signals in a plurality of uplink frequency bands are received by the remote unit. Each of the remote units further comprises an uplink splitting-filtering means for separating the received uplink RF signals into a plurality of uplink RF-groups by frequency band; a plurality of uplink-signal-conditioning assemblies for performing individual uplink-signal-conditioning on each of the uplink RF-groups; an uplink filtering-combining means for combining the individual-conditioned uplink RF-groups into an uplink RF-part; and an uplink RF-to-optical converter for converting the uplink RF-part to an uplink optical signal. Each of the remote units may be further coupled to an auxiliary antenna by an RF-switching means, whereby downlink RF signals in a TDD frequency band from the remote unit are transmitted to, and uplink RF signals in the TDD frequency band are received at the remote unit from this TDD antenna by actuating the RF-switching means. The RF-switching means can be provided by an RF-(Transmit/Receive)switch coupled to a downlink power-detect means, whereby it is actuated according to the power level of the downlink RF signals in the TDD frequency band as determined by the downlink power-detect means. Alternatively, a downlink RF-switch and an uplink RF-switch can be separately implemented along a downlink TDD RF-path and an uplink TDD RF-path, and further coupled to a downlink power-detect means in a remote unit. By detecting the power level on the downlink TDD RF-path, the downlink power-detect means enables the downlink TDD signals to be transmitted, along with the downlink (FDD) RF signals in other downlink frequency bands, to the downlink antenna; while permitting the uplink TDD signals to be received, along with the uplink (FDD) RF signals in other uplink frequency bands, from the uplink antenna when there is no downlink transmission.
The multiband distributed wireless communications system of the present invention may further comprise a frequency-translation means for performing various frequency-translations on downlink and uplink RF signals, to allow for feasible separation of downlink RF signals into downlink RF-groups by frequency band and to prevent the interference effects and intermodulation products amongst different (downlink and uplink) frequency bands. By way of example, the frequency-translation means may comprise a global-tone mixer (coupled to a global-tone generator) in RF-communication with the downlink RF-combining means at the main unit, so as to perform one or more first frequency-translations on one or more downlink RF-sets and thereby place the downlink RF-sets in disjoint frequency bands that are sufficiently far apart to allow for economical separation of downlink RF signals in different bands by way of RF-filtering. The frequency-translation means may further comprise multiple remote global-tone mixers coupled to the remote units, such that there are one or more remote global-tone mixers in each of the remote units, for performing one or more second frequency-translations and thereby placing the downlink RF-groups back into their original frequency bands respectively. (The remote global-tone mixers may be coupled to a remote global-tone generator, which is substantially the same as the one used in the main unit, or receive a global-tone signal from the main unit.) The frequency-translation means may also be in the form of one or more downlink-local-tone mixers (coupled to a downlink-local-tone generator), in RF-communication with at least one of the downlink-signal-conditioning assemblies in a remote unit. The downlink-local-tone mixers serve to place one or more downlink RF-groups in one or more intermediate frequency bands where the downlink-signal-conditioning on these downlink RF-groups can be more effectively performed, and subsequently place these downlink RF-groups back into their respective original frequency bands. The frequency-translation means may further be in the form of one or more uplink-local-tone mixers (coupled to an uplink-local-tone generator), in RF-communication with one or more uplink-signal-conditioning assemblies in a remote unit. The uplink-local-tone mixers likewise serve to place one or more uplink RF-groups in one or more intermediate frequency bands, such that the uplink-signal-conditioning on these uplink RF-groups can be performed more effectively. Moreover, the frequency-translation means can be in the form of a combination of global-tone and local-tone mixers (and other frequency-translation means known in the art) implemented in the main unit and the remote units, for performing various frequency-translations on downlink and uplink RF signals, so as to best facilitate the distribution of multiband RF signals.
The multiband distributed wireless communications system of the present invention may further comprise a gain-calibration means for carrying out an end-to-end gain calibration (initially or when there is no transmission of downlink RF signals), thereby setting a prescribed gain for each of the downlink RF-groups. As a way of example, the gain-calibration means may be provided by a calibration-tone generator in RF-communication with the downlink RF-combining means in the main unit that injects a calibration tone. The frequency of the calibration tone is set to lie within the frequency band of each downlink RF-group to be calibrated. To maintain the prescribed gain against temperature changes and other effects, one or more gain-control-signal combiners can be implemented in the main unit, for injecting a gain-control-signal (e.g., a pilot or FSK signal) to each of the downlink RF-parts to be transmitted to the remote units. The gain-control-signal is set at a frequency outside of any of the downlink frequency bands used by the wireless communications networks (and frequency-translated bands). Each of the remote units further comprises a downlink RF-splitting means coupled with a downlink gain-control element, for detecting and thereby using the gain-control-signal to maintain the desired gain for each of the downlink RF-groups.
The multiband distributed wireless communications system of the present invention may further comprise one or more expansion units, serving as intermediate hubs for linking the main unit to additional remote units. As a way of example, an expansion unit can be configured in the form of a downlink assembly and an uplink assembly. The downlink assembly may include a downlink optical-to-RF converter for converting a downlink optical signal delivered to the expansion unit to an RF signal; a downlink RF-amplifier for amplifying the RF signal; a downlink RF-to-optical converter for converting the amplified RF signal to an optical signal; and an optical-splitting means for splitting the optical signal to multiple secondary-optical-signals, which are subsequently transmitted to additional remote units (and/or one or more lower-level expansion units). Alternatively, the RF signal may first be split into multiple secondary-RF-signals by an appropriate RF-splitting means, which are then converted to multiple secondary-optical-signals. The uplink assembly may include an optical-combining element for combining multiple uplink optical signals arriving at the expansion unit into a combined optical signal; an uplink optical-to-RF converter for converting the combined optical signal to a combined RF signal; an uplink RF-amplifier for amplifying the combined RF signal; and an uplink RF-to-optical converter for converting the combined RF signal to an optical signal, which is further transmitted to the main unit (or a higher-level expansion unit). Alternatively, the uplink optical signals may first be converted to multiple uplink RF signals, which are subsequently combined into a combined RF signal by a suitable RF-combining means. The exemplary embodiment described above provides only one of many embodiments of a multiband distributed wireless communications system according to the present invention. Those skilled in the art will recognize that a variety of multiband distributed wireless communication systems can be constructed according to the principle of the present invention, and various means and methods can be devised to perform the designated functions in an efficient/equivalent manner. Moreover, various changes, substitutions, and alternations can be made herein without departing from the principle and the scope of the invention. For instance, the combined downlink RF signal may be first converted to a combined optical signal at the main unit, which is subsequently split into multiple downlink optical signals by an appropriate optical-splitting means. Likewise, multiple uplink optical signals received at the main unit may be first combined into a combined optical signal by a suitable optical-combining means, which is then converted to a combined RF signal to be transmitted to the wireless communications networks. Various RF-splitting means, RF-combining means, RF-filtering means, RF-switching means, and frequency-translation means depicted in the above embodiments can be provided by RF-splitters, RF-combiners, RF-filters, RF-switches, RF-circulators, power combiners, duplexers, triplexers (and other suitable multiplexers), frequency mixers and multipliers known in the art. Moreover, a wavelength-division-multiplexing (WDM) filter may be used to transmit a pair of downlink and uplink optical signals along a single optical fiber, thereby reducing the number of optical fibers to be deployed in the system. Additionally, the pilot (or FSK) signals employed in the present invention for the purpose of gain-control can be further utilized to establish digital communications amongst the main unit and remote units.
As such, the method and system for distributing multiband wireless communication signals according to the present invention provide many advantages over the prior art systems, summarized as follows:
1. The use of separate downlink and uplink antennae dedicated to each remote unit provides a simple and efficient way to transmit and receive RF signals in multiple frequency bands upon downlink and uplink. Such an implementation is especially effective when dealing with multiple (e.g., more than two) wireless frequency bands, including intertwined bands (such as cellular and iDEN bands). Moreover, having separate uplink and downlink antennae enables the reception of uplink RF signals and the transmission of downlink RF signals to be spatially separated in the present invention. Such a spatial separation creates a propagation loss between the transmit (downlink) and receive (uplink) antennae, which helps protect the sensitive uplink receivers from being desensitized by strong downlink RF signals and/or by downlink intermodulation products that fall into one or more uplink frequency bands.
2. By separating downlink (or uplink) RF signals according to frequency band in each remote unit, RF signals in different frequency bands are individually conditioned (e.g., filtered and amplified), before being recombined to be transmitted to a dedicated downlink antenna (or after being received from a dedicated uplink antenna).
3. Various frequency translations employed in the present invention make it possible to separate downlink RF signals into downlink RF-groups by frequency band using feasible means (such as RF-filtering), such that these downlink RF-groups can be individually conditioned (e.g., filtered and amplified) before being transmitted to a downlink antenna in a remote unit. The frequency translations can also be effectively utilized to prevent the interference effects and intermodulation products amongst different (downlink and uplink) frequency bands. As such, these frequency translations effectively facilitate the transportation and distribution of wireless RF signals in multiple frequency bands, and are particularly desirable when dealing with RF signals in adjacent (and/or intertwined) frequency bands.
4. The gain for each of the downlink RF-groups is individually calibrated and established at a prescribed level, and further maintained over temperature changes and other extraneous effects.
5. The present invention support both FDD and TDD protocols in a simple and flexible way. The use of an RF-switching means to couple a TDD antenna to a remote unit, or the implementation of appropriate RF-switches coupled with an RF power-detect means in a remote unit, provides a simple and effective way of transmitting and receiving TDD signals. Such implementations prevent the noise transmitted on the downlink from desensitizing the reception on the uplink (since the uplink and downlink share the same frequency band for TDD protocols).
6. The employment of a novel hybrid star/cascaded architecture provides a modular, flexible and efficient way of distributing multiband wireless RF signals.
7. The use of a pilot (or an FSK) signal not only provides an effective way of maintaining the desired gain for each of the downlink RF-groups, it can also be utilized to establish an effective communication link between the main unit and the remote units.
All in all, the present invention provides an efficient, flexible, and economical way of transporting and distributing wireless communication signals in multiple (adjacent, intertwined, or otherwise) frequency bands.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.